1. Field of the Invention
The present invention relates to a drive-force distribution controller for a four-wheel-drive vehicle, and more particularly to a drive-force distribution controller for a four-wheel-drive vehicle which can distribute proper drive forces to front and rear wheels in accordance with traveling conditions of a vehicle to thereby improve traveling stability and steering feel.
2. Description of the Related Art
Conventionally, there has been known a drive-force distribution controller for a four-wheel-drive vehicle which variably controls the engagement force of a torque distribution clutch in accordance with the difference in rotational speed between front and rear wheels. FIG. 1 shows an exemplary control map used in such a drive-force distribution controller for a four-wheel-drive vehicle. In FIG. 1, the vertical axis represents engagement force T, and the horizontal axis represents rotational speed difference xcex94N between front and rear wheels.
At the time of acceleration and starting on a so-called low-xcexc road such as a snow-covered road or an icy road, acceleration or starting can be effected in a stable manner if the engagement force T is rendered large at the time of acceleration or starting through employment of a mapping curve B indicated by a chain line in FIG. 1.
However, increased engagement force makes it difficult to absorb a rotational speed difference produced between the front and rear wheels while a vehicle travels around a tight corner or is being parked or put into a garage with a large steering angle, resulting in occurrence of a so-called tight-corner braking phenomenon (in which turning becomes difficult as if brakes were being applied), and possible stalling of the engine.
This problem may be solved though employment of the mapping curve B which sharply increases the engagement force as the rotational speed difference xcex94N increases, and a mapping curve C which moderately increases the engagement force as the rotational speed difference xcex94N increases as shown in FIG. 1. These mapping curves B and C are selectively used depending on whether the rotational speed difference xcex94N between the front and rear wheels is produced due to starting of the vehicle on a low-xcexc road or acceleration, or due to traveling around a tight corner. However, it has been difficult to judge whether the rotational speed difference xcex94N between the front and rear wheels is produced due to starting of the vehicle on a low-xcexc road or acceleration, or due to traveling around a tight corner. In order to solve this difficulty, there has been proposed a technique in which steering angle is detected by use of a steering angle sensor, and when a steering angle greater than a predetermined value is detected, a vehicle is judged to be traveling around a tight corner or in a tight-corner traveling mode. Further, there has been proposed a technique in which the amount by which an accelerator is depressed is detected by use of an accelerator sensor, and when an accelerator depression amount greater than a predetermined value is detected, the vehicle is judged to be accelerating or in an acceleration mode.
However, provision of the steering sensor and the accelerator sensor increases cost, which is undesirable.
Therefore, when use of the steering sensor and the accelerator sensor must be avoided, the conventional drive-force distribution controller for a four-wheel-drive vehicle uses a mapping curve A which is shown by a solid line in FIG. 1 and which has a slope between that of the mapping curve B, which sharply increases the engagement force with increase in the rotational speed difference xcex94N, and that of the mapping curve C, which moderately increases the engagement force with increase in the rotational speed difference xcex94N.
However, since the mapping curve A used in the conventional drive-force distribution controller for a four-wheel-drive vehicle is between the mapping curves B and C, a large engagement force cannot be obtained at the time of starting on a low-xcexc road or at the time of acceleration, so that wheels which receive the distributed drive force easily slip or spin out. Further, the above-mentioned tight-corner braking phenomenon easily occurs when the vehicle travels around a tight corner at low speed or is parked or put into a garage.
That is, the conventional drive-force distribution controller for a four-wheel-drive vehicle cannot determine whether a rotational speed difference xcex94N is produced between the front and rear wheels due to either acceleration or starting, or due to traveling around a tight corner, and therefore cannot finely control the engagement force of the torque distribution clutch in accordance with the traveling conditions of the four-wheel-drive vehicle. Accordingly, the drive-force distribution controller cannot improve traveling stability and steering feel.
In view of the foregoing, an object of the present invention is to provide a drive-force distribution controller for a four-wheel-drive vehicle which can finely control the engagement force of a torque distribution clutch in accordance with the traveling conditions of the four-wheel-drive vehicle to thereby improve traveling stability and steering feel.
The present invention provides a drive-force distribution controller for a four-wheel-drive vehicle in which drive force produced by a prime mover is transmitted directly to front or rear wheels and is transmitted to the remaining wheels via a torque distribution clutch, and the engagement force of the torque distribution clutch is controlled in accordance with traveling conditions of the vehicle. The drive-force distribution controller comprises a calculation unit for calculating variation per unit time in rotational speed difference between the front wheels and the rear wheels; and a control unit for controlling the engagement force such that the engagement force increases as the variation per unit time in the rotational speed difference increases.
The calculation unit calculates variation per unit time in the rotational speed difference between the front wheels and the rear wheels; i.e., acceleration of the rotational speed difference. The acceleration of the rotational speed difference becomes large when the vehicle starts on a low-xcexc road, such as a snow-covered road or an icy road, or starts abruptly, and becomes small when the vehicle travels around a tight corner or is parked or put into a garage with a large steering angle.
The control unit controls the torque distribution clutch such that the engagement force increases as the variation per unit time in the rotational speed difference increases, as calculated by the calculation unit.
In other words, the control unit increases the engagement force when the vehicle starts on a low-xcexc road, such as a snow-covered road or an icy road, or starts abruptly, because the acceleration of the rotational speed difference becomes large in such a state.
Accordingly, the ratio of distribution of drive force to wheels which are not connected directly to the prime mover (i.e., wheels which receive a portion of the drive force) can be increased, which enables stable starting and acceleration while preventing slippage of the wheels.
In contrast, the control unit decreases the engagement force when the vehicle travels around a tight corner or is parked or put into a garage with a large steering angle, because the acceleration of the rotational speed difference becomes small in such a state.
Accordingly, the rotational speed difference between the front and rear wheels can be absorbed, whereby occurrence of the above-mentioned tight-corner braking phenomenon can be prevented.
Preferably, the control unit controls the engagement force in accordance with the rotational speed difference, as well as variation per unit time in the rotational speed difference. More preferably, the control unit comprises a control map for determining the engagement force in accordance with the rotational speed difference and variation per unit time in the rotational speed difference.
Preferably, the drive force distribution controller further comprises a sensor for detecting the difference between rotational speed on the input side of the torque distribution clutch and rotational speed on the output side of the torque distribution clutch, and the calculation unit calculates variation per unit time in the rotational speed difference detected by the sensor.
More preferably, the sensor comprises first and second annular members which are disposed to rotate together with one of input-side and output-side members of the torque distribution clutch and which are provided with sensing teeth formed on their outer circumferential surfaces at a predetermined pitch such that a phase difference is provided between the teeth of the first annular member and the teeth of the second annular member; and a pair of sensing heads disposed to rotate together with the other of the input-side and output-side members of the torque distribution clutch and torque the sensing teeth of the first annular member and the sensing teeth of the second annular member, respectively.
The present invention further provides a drive-force distribution controller for a four-wheel-drive vehicle in which drive force produced by a prime mover is transmitted directly to front wheels and is transmitted to rear wheels via a torque distribution clutch, and the engagement force of the torque distribution clutch is controlled in accordance with traveling conditions of the vehicle. The drive-force distribution controller comprises a first judgment unit for judging which is greater; the rotational speed of the front wheels or the rotational speed of the rear wheels; a second judgment unit which is enabled when the first judgment unit has judged that the rotational speed of the front wheels is greater than the rotational speed of the rear wheels, in order to judge whether the acceleration of the vehicle is greater than a predetermined level; a first setting unit for setting the engagement force to a relatively large first value when the second judgment unit has judged that the acceleration of the vehicle is greater than the predetermined level; a second setting unit for setting the engagement force to a second value smaller than the first value when the second judgment unit has judged that the acceleration of the vehicle is not greater than the predetermined level; and a third setting unit for setting the engagement force to a third value smaller than the first value but greater than the second value when the first judgment unit has judged that the rotational speed of the front wheels is less than the rotational speed of the rear wheels.
The first judgment unit judges which is greater; the rotational speed of the front wheels or the rotational speed of the rear wheels.
The four-wheel-drive vehicle designed on the basis of front wheel drive in which the drive force generated by the prime mover is transmitted directly to the front wheels has the following characteristics. When the vehicle is in a tight-corner mode (when the vehicle travels at low speed around a tight corner, or is being parked or put into a garage) or in an acceleration mode (when the vehicle accelerates or when the vehicle starts on a low-xcexc road such as a snow-covered road or an icy road), the rotational speed of the front wheels becomes greater than that of the rear wheels (a forward-rotation mode). In contrast, when the vehicle in a reverse-rotation mode (when braking or engine brake is effected), the rotational speed of the rear wheels becomes greater than that of the front wheels. Therefore, it is possible to judge whether the vehicle is in the forward-rotation mode or the reverse-rotation mode through judgment as to which is greater, the rotational speed of the front wheels or the rotational speed of the rear wheels.
When the first judgment unit has judged that the rotational speed of the front wheels is greater than the rotational speed of the rear wheels, the second judgment unit judges whether the acceleration of the vehicle is greater than a predetermined level.
As described above, the forward-rotation mode includes two modes; i.e., the tight-corner mode and the acceleration mode. Since the acceleration of the vehicle in the tight-corner mode is smaller than that is the acceleration mode, it is possible to judge whether the vehicle is in the tight-corner mode or the acceleration mode through judgment as to whether the acceleration of the vehicle is greater than a predetermined level.
The first setting unit sets the engagement force to a relatively large first value when the second judgment unit has judged that the acceleration of the vehicle is greater than the predetermined level; i.e., when the vehicle is in the acceleration mode.
That is, when the vehicle starts on a low-xcexc road or accelerates at an acceleration greater than the predetermined level, the engagement force of the torque distribution clutch can be increased in order to increase the ratio of distribution to the rear wheels of the drive force generated by the prime mover. Therefore, starting and acceleration can be effected in a stable manner, while slippage of the front wheels is prevented.
The second setting unit sets the engagement force to a second value smaller than the first value when the second judgment unit has judged that the acceleration of the vehicle is not greater than the predetermined level.
That is, when the vehicle travels at low speed around a tight corner, or is parked or put into a garage, the engagement force of the torque distribution clutch can be decreased in order to absorb the rotational speed difference between the front and rear wheels. Thus, the above-described tight-corner braking phenomenon can be prevented.
Further, the third setting unit sets the engagement force to a third value smaller than the first value but greater than the second value when the first judgment unit has judged that the rotational speed of the front wheels is less than the rotational speed of the rear wheels.
That is, when the rotational speed of the rear wheels becomes greater than the rotational speed of the front wheels due to, for example, deceleration of the vehicle caused by means of braking or engine brake, the engagement force is set to the third value smaller than the first value but greater than the second value in order to prevent slippage of the front wheels to thereby improve traveling stability.
Preferably, each of the first to third setting units sets the engagement force in consideration of the rotational speed difference between the front and rear wheels. More preferably, each of the first to third setting units sets the engagement force by use of a control map.
The present invention further provides a drive-force distribution controller for a four-wheel-drive vehicle in which drive force produced by a prime mover is transmitted directly to rear wheels and is transmitted to front wheels via a torque distribution clutch, and the engagement force of the torque distribution clutch is controlled in accordance with traveling conditions of the vehicle. The drive-force distribution controller comprises a first judgment unit for judging which is greater; the rotational speed of the front wheels or the rotational speed of the rear wheels, a first setting unit for setting the engagement force to a relatively large first value when the first judgment unit has judged that the rotational speed of the front wheels is less than the rotational speed of the rear wheels; and a second setting unit for setting the engagement force to a second value smaller than the first value when the first judgment unit has judged that the rotational speed of the front wheels is greater than the rotational speed of the rear wheels, wherein the second value increases with the speed of the vehicle.
The first judgment unit judges which is greater, the rotational speed of the front wheels or the rotational speed of the rear wheels.
The four-wheel-drive vehicle designed on the basis of rear wheel drive in which the drive force generated by the prime mover is transmitted directly to the rear wheels has the following characteristics. When the vehicle is in an acceleration mode (when the vehicle accelerates or when the vehicle starts on a low-xcexc road such as a snow-covered road or an icy road), the rotational speed of the rear wheels becomes greater than that of the front wheels (a forward-rotation mode). In contrast, when the vehicle is in a reverse-rotation/tight-corner mode (when the vehicle travels at low speed around a tight corner, or is parked or put into a garage; or when braking or engine brake is effected), the rotational speed of the front wheels becomes greater than that of the rear wheels. Therefore, it is possible to judge whether the vehicle is in the forward-rotation mode or the reverse-rotation/tight-corner mode through judgment as to which is greater; the rotational speed of the front wheels or the rotational speed of the rear wheels.
The first setting unit sets the engagement force to a relatively large first value when the first judgment unit has judged that the rotational speed of the rear wheels is greater than the rotational speed of the front wheels; i.e., when the vehicle is in the acceleration mode.
That is, when the vehicle starts on a low-xcexc road or accelerates at an acceleration greater than the predetermined level the engagement force of the torque distribution clutch can be increased in order to increase the ratio of distribution to the front wheels of the drive force generated by the prime mover. Therefore, starting and acceleration can be effected in a stable manner, while slippage of the rear wheels is prevented.
When the first judgment unit has judged that the rotational speed of the front wheels is greater than the rotational speed of the rear wheels, the second setting unit sets the engagement force to a second value which is smaller than the first value and which increases with the speed of the vehicle.
That is, when the vehicle travels at low speed around a tight corner, or is being parked or put into a garage, the engagement force of the torque distribution clutch can be decreased in order to absorb the rotational speed difference between the front and rear wheels. Thus, the above-described tight-corner braking phenomenon can be prevented.
Further, when the rotational speed of the front wheels becomes greater than the rotational speed of the rear wheels due to, for example, deceleration of the vehicle caused by means of braking or engine brake, the engagement force is set to the second value which is smaller than the first value and which increases with the speed of the vehicle. Thus, slippage of the rear wheels is prevented to thereby improve traveling stability.
Preferably, each of the first and second setting units sets the engagement force in consideration of the rotational speed difference between the front and rear wheels. More preferably, each of the first and second setting units sets the engagement force by use of a control map.